I shouldn't be writing to you actually because I have difficulties with my amost 8 year son, too when it comes to homework--he does it but it can take him up to 2 hours what should take him about 30 minutes and then in my perspective he should be turning in better work than he does (he goes to a German school).
Though my sister has told me of a co-workers of hers who has a daughter who was never on time to go to school. She did talk with a psychologist about the problem and was told that she should put the whole responsibility on the dauther. If she must come too late to school then so be it but she had to take the consequenses. Well, it worked for her that after a few times being to late she cut out the behaviour.
I also have talke with another friend of mine who had similar experience with her son and she only makes sure that he does some homework but she will not look at it nor get involved. It'ts between the kid and the teacher.
Having all that said I think you should go and talk to the teacher and tell them that you are well aware of his problems but that it has come to the point where you are beyond any progress and therefore you will totally step back, let him not do his work and come late to school but support the teacher with anything they impose on him for "punishment." That way the teacher knows you care and at the same time you will have a bit peace in the family again.
I will take the same aproach for the rest of the school year. My son has a homework book in which I make notes like he did not write down his homework etc. So the teacher knows that I'm on top of what is going on but at the same time am not involved. See what kind of communication the teacher will prefer if they go with that approach.
I guess schoolwork is like food, we can't force them to do it (eat it) it's their way of showing they have power, too. For me it's hard to see that my son is working beyond his potential but I guess we can't prevent our kids from making mistakes, they want to have exeriences on their own, just like our parents coudn't help us with our exeriences.